The compressor in a gas turbine engine is designed to operate over a wide range of gas flow and rotational speed while delivering a stable pressure ratio. During the engine start phase, the compressor must be capable of such stable operation at low rotational speeds.
Compressor surge is an undesirable phenomenon in which the compressor operates in an unstable manner. Surge is manifested as an operating condition having a compressor back pressure that is higher than the pressure capability of the compressor, which in turn causes the previously steady compressor flow to break down and reverse direction.
This flow breakdown changes the forces applied to the compressor rotor and in so doing causes large flow fluctuations through the compressor. This can result in high levels of vibration and may cause significant mechanical damage to the engine.
Rotating stall and stall flutter are other compressor instabilities which are undesirable and have the potential to cause engine damage. In general, rotating stall is more likely to occur at low engine speeds, whereas surge is more likely to occur at high engine speeds. Collectively, these are types of compressor instability. (Note that in the USA, compressor surge is known as compressor stall).
These compressor instabilities are fundamentally caused by flow separation in a compressor blade row. The extent and severity of the flow separation (radially and axially) and the blade's response dictate the engine's response and hence the name given to the event.
The compressor operating conditions associated with instability are represented on a compressor performance map as a stability boundary commonly known as a surge, or stall, line, i.e. a boundary between stable compressor operation and unstable (surge/stall) operation. The margin between this line and the compressor operating point is commonly referred to as a surge margin and may be defined as:
      S    ⁢                  ⁢    M    =            (                        P          ⁢                                          ⁢                      R            surge                          -                  P          ⁢                                          ⁢                      R            working                              )              P      ⁢                          ⁢              R        working            
where: PRsurge=pressure ratio on the surge line at a corrected mass flow rate, ωc;                PRworking=pressure ratio on the working line at mass flow rate, {dot over (m)}.        
Consequently, conventional ground starting schedules for aircraft gas turbine engines are arranged to provide the engine with optimum conditions for the ‘light up’ phase whilst avoiding compressor instability by keeping the operating point well away from the surge line.
Normally a ground start fuel control schedule is arranged to give the best chance of light up while remaining clear of compressor stability boundaries, with margin to allow for the worst case stability margin in the life of the engine.
Once the engine is operating above the idle speed, the engine operating control strategy maintains stable compressor operation by ensuring that the compressor working line is sufficiently separated from the surge line to accommodate degradation of the surge margin over the life of the engine. Surge margin “aids” such as handling bleeds and variable stator vanes in the engine are controlled by the engine's control system to allow surge free operation through life.
The provision of high levels of surge margin is often required in order to ensure that stable operation of the compressor is possible over its full expected operating range.
Increasing the surge margin provides certainty that the compressor operating point remains away from the surge line but tends to compromise other attributes such as compressor cost, weight and efficiency.